"While it is very likely we will see more imported cases, it is too early to say if this will happen on a regular basis," said Jen Kates, vice president and director of global health and HIV policy at the Kaiser Family Foundation.

"Given the attention to this and precautions, [it is] unlikely to become a regular health occurrence, particularly if public health guidance is followed."

In a prepared statement, a spokeswoman for the Texas Department of State Health Services said that officials are trying to find as many as 100 people who may have come in contact with the Dallas-based Ebola patient, saying "out of an abundance of caution, we're starting with this very wide net, including people who have had even brief encounters with the patient or the patient's home. The number will drop as we focus in on those whose contact may represent a potential risk of infection," spokeswoman Carrie Williams, said.

In West Africa, the disease has been transmitted widely because of inadequate medical care, lack of equipment to keep medical personnel safe, and burial practices that expose mourners to bodily fluids of the deceased.

Years of civil war and corruption have also made West Africans distrustful of their government and other authorities, so many people hid from rather than approached health care workers and Ebola clinics. (Related: "U.S. Ebola Aid Could Tamp Down Fear in West Africa.")

The illness has infected more than 7,100 people in the region, killing 3,300, according to the World Health Organization, which acknowledges the figures probably dramatically undercount actual cases.

In the United States, conditions are starkly different, public health officials say, making it much less likely that the disease will spread widely. American hospitals are prepared for patients, with adequate supplies of gloves, gowns, and cleaning equipment—to limit contact with patient's bodily fluids—and a ready flow of knowledgeable doctors and nurses.

It's also easier to trace the people who come into contact with a patient in the United States, as has been done with the Dallas patient, and keep them from infecting more people. So-called contact tracing in West Africa is difficult because many areas lack basic street addresses, and because there is a shortage of personnel to follow up on those who had contact with patients.

Tracking Contacts

It's hard to identify Ebola in its early stages because the initial symptoms of the disease—fever, diarrhea, vomiting, and aches—are the same as for many other illnesses, including malaria, which is also common in West Africa.

That's why the Dallas patient was able to leave Liberia on September 19 without any indication that he would soon be ravaged by Ebola. The West African nations where the virus is raging are supposed to test outgoing passengers for fever, to make sure that no one with Ebola symptoms gets on a plane and spreads the disease. (See "Doctors and Nurses Risk Everything to Fight Ebola in West Africa.")

A blood test that scans for the virus's DNA is considered the definitive diagnostic, but viral loads are not high enough to confirm the diagnosis until well after symptoms have appeared, Frieden said Tuesday. Such testing can be done at the CDC's headquarters in Atlanta and in several other labs around the U.S.

Symptoms usually appear about a week after infection; patients are not contagious unless they are symptomatic, according to the CDC.

The people who came into close contact with the Dallas patient while he was sick, from September 24 until he was isolated in a hospital room on September 28, are at risk for the virus.

Public health officials are tracking 12 to 18 people in Dallas who had contact with the patient, including 5 children who have been asked to stay home from school. Medical personnel plan to check their temperature twice a day and to look out for other symptoms.

Texas Governor Rick Perry took to the airwaves Wednesday to assure residents that they are safe.

"There is no place that has better ability to address this than in Texas," he said at a midday, televised news conference. "We wish it were somewhere else. But the fact is it's here, and we have an extraordinary team of men and women passionately and compassionately delivering care to this individual and doing everything possible to make sure that the citizens of this state are fully protected from this virus."

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A relative grieves as a Red Cross burial team prepares to remove the body of an Ebola victim in Monrovia, Liberia, last month.

Photograph by Daniel Berehulak, The New York Times

Months of Preparation

American hospitals have been preparing since midsummer for the possibility of having to care for an Ebola patient, but Tuesday's news took the development out of the theoretical realm. Since then, there have been 12 false alarms—patients suspected of having Ebola who did not have the disease, the CDC said.

The CDC has been advising hospitals for several months to prepare for patients with Ebola, though each hospital is preparing slightly differently.

At Texas Health Presbyterian Hospital, staff ran a drill last week to prepare for possible patients, according to the hospital's epidemiologist, Edward Goodman.

At Montefiore, signs in the emergency department ask patients to let caregivers know if they've recently traveled to the West African nations of Liberia, Sierra Leone, or Guinea.

At Beth Israel Deaconess Medical Center in Boston, doctors and nurses are on high alert for patients with symptoms like fever, said Sharon Wright, director of infection control and epidemiology. "Right now we're depending on astute clinicians" to detect potential patients, she said.

Wright expects many more false alarms than actual cases of Ebola at Beth Israel, a Harvard-affiliated hospital.

The hospital has established a separate space for isolating and treating suspected patients, where there is room for the extra equipment and trash disposal required, and where other hospital patients and family members won't need to worry about exposure, she said. A room off the main lab has been dedicated to studying the blood of potential patients, without risking contamination of other equipment, she said.

"In the end, all health care providers at any medical center are prepared to take care of any patient, and that's the best we can do," Wright said. "We're just trying to make sure we're protecting our staff and family members and other patients at the same time."